Communities, populations and individuals of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi in the phylum Glomeromycota are found globally in most vegetation types, where they form a mutualistic symbiosis with plant roots. Despite their wide distribution, only relatively few species are described. The taxonomy is based on morphological characters of the asexual resting spores, but molecular approaches to community ecology have revealed a considerable unknown diversity from colonized roots. Although the lack of genetic recombination is not unique in the fungal kingdom, arbuscular mycorrhizal fungi are probably ancient asexuals. The long asexual evolution of the fungi has resulted in considerable genetic diversity within morphologically recognizable species, and challenges our concepts of individuals and populations. This review critically examines the concepts of species, communities, populations and individuals of arbuscular mycorrhizal fungi.     

Interactions between Lotus japonicus genotypes and arbuscular mycorrhizal fungi

Abstract

Interactions between three genotypes (Ljsym 71-1, Ljsym 71-2 and Ljsym 72) of Lotus japoicus and one isolate from each of four species of arbuscular mycorrhizal fungi (Glomus sp. R-10, Glomus intraradices, Glomus etunicatum, and Gigaspora margarita) were investigated and compared with the wild-type ‘Gifu’ B-129. All the three genotypes showed no or defective internal colonization after inoculation with these AM fungi. In Ljsym72 mutant, the AM fungi produced deformed appressoria on the root surface, but failed to form any internal structures (internal hyphae, arbuscules and vesicles) except only in Glomus intraradices. The Ljsym71-1 and Ljsym71-2 mutants had more deformed appressoria and occasionally formed internal hyphae, arbuscules and vesicles, depending on AM fungi used. Wild-type ‘Gifu’ (nod⁺myc⁺) plants had typical colonization. The colonization of mutants by several fungi varied and provides a basis for studying recognition and compatibility between plants and mycorrhizal fungal species. These mutants also will be useful in studies of the genetics of the symbiosis between plant species and AM fungi.     

丛枝菌根真菌四个中国新记录种

从梅和滨海植物根区土壤中分离得到丛枝菌根真菌4个中国新记录,即沙生球囊霉Glomus arenarium、莫顿球囊霉G. mortonii、三壁球囊霉G. trimurales和苍耳管柄囊霉Funneliformis xanthium。对其形态特征进行重新描述和图解。这4个种的标本保存在中国科学院菌物标本馆（HMAS）和厦门市园林植物园标本馆中。     

Community structure of arbuscular mycorrhizal fungi in undisturbed vegetation revealed by analyses of LSU rDNA sequences

Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with plant roots and are found in most ecosystems. In this study the community structure of AMF in a clade of the genus Glomus was examined in undisturbed costal grassland using LSU rDNA sequences amplified from roots of Hieracium pilosella. Roots were sampled from May to November along eight 30-m transects, 30-120 m apart. Phylogenetic analysis of the sequences revealed 11 phylogenetic clusters within the clade of Glomus. The phylogenetic clusters were patchily distributed within the area; time had no influence on the distribution pattern. The dominant cluster covered up to 10 m along the transect, whereas other clusters formed what can be interpreted as small individual mycelia. Four of the phylogenetic clusters included known species; the other clusters, including the dominant sequence types, were unknown. The dominant phylogenetic cluster enclosed nine haplotypes, and analyses of genetic diversity of this phylogenetic cluster showed that the total diversity could be found within single root fragments, suggesting that the multiple sequences were derived from a single individual.     

Clonal diversity and population genetic structure of arbuscular mycorrhizal fungi (Glomus spp.) studied by multilocus genotyping of single spores

A nested multiplex PCR (polymerase chain reaction) approach was used for multilocus genotyping of arbuscular mycorrhizal fungal populations. This method allowed us to amplify multiple loci from Glomus single spores in a single PCR amplification. Variable introns in the two protein coding genes GmFOX2 and GmTOR2 were applied as codominant genetic markers together with the LSU rDNA. Genetic structure of Glomus spp. populations from an organically and a conventionally cultured field were compared by hierarchical sampling of spores from four plots in each field. Multilocus genotypes were characterized by SSCP (single stranded conformation polymorphism) and sequencing. All spore genotypes were unique suggesting that no recombination was taking place in the populations. There were no overall differences in the distribution of genotypes in the two fields and identical genotypes could be sampled from both fields. Analysis of gene diversity indicated that Glomus populations are subdivided between plots within each field. There were however, no subdivision between the fields.     

The liverwort Marchantia foliacea forms a specialized symbiosis with arbuscular mycorrhizal fungi in the genus Glomus

Microscopic evidence suggests that fungi forming endosymbioses with liverworts in the Marchantiales are arbuscular mycorrhizal (AM) fungi from the Glomeromycota. Polymerase chain reaction amplification of ribosomal sequences confirmed that endophytes of the New Zealand liverwort, Marchantia foliacea, were members of the genus Glomus. Endophytes from two Glomus rDNA phylotypes were repeatedly isolated from geographically separated liverwort samples. Multiple phylotypes were present in the same liverwort patch. The colonizing Glomus species exhibited substantial internal transcribed spacer sequence variation within phylotypes. This work suggests that certain liverwort species may serve as a model for studying DNA sequence variation in colonizing AM phylotypes and specificity in AM-host relationships.     

Colonization of pennycresses (Thlaspi spp.) of the Brassicaceae by arbuscular mycorrhizal fungi

Members of the Brassicaceae are generally believed to be non-mycorrhizal. Pennycress (Thlaspi) species of this family from diverse locations in Slovenia, Austria, Italy and Germany were examined for their colonisation by arbuscular mycorrhizal fungi (AMF). Meadow species (T. praecox, T. caerulescens and T. montanum) were sparsely but distinctly colonised, as indicated by the occurrence of intraradical hyphae, vesicles, coils, and occasionally arbuscules. Species from other locations were poorly colonised, but arbuscules were not discernible. The genus Thlaspi comprises several heavy metal hyperaccumulating species (T. caerulescens, T. goesingense, T. calaminare, T. cepaeifolium). All samples collected from heavy metal soils were at best poorly colonized. Thus the chance is small to find a "hypersystem" in phytoremediation consisting of an AM fungus which prevents the uptake of the major part of the heavy metals and of a Thlaspi species which effectively deposits the residual heavy metals inevitably taken up into its vacuoles. In two different PCR approaches, fungal DNA was amplified from most of the Thlaspi roots examined, even from those with a very low incidence of AMF colonization. Sequencing of the 28S- and 18S-rDNA PCR-products revealed that different Thlaspi field samples were colonized by Glomus intraradices and thus by a common AM fungus. However, none of the sequences obtained was identical to any other found in the present study or deposited in the databanks, which might indicate that a species continuum exists in the G. intraradices clade. An effective colonization of Thlaspi by AMF could not be established in greenhouse experiments. Although the data show that Thlaspi can be colonized by AMF, it is doubtful whether an effective symbiosis with the mutual exchange of metabolites is formed by both partners.     

Vesicular arbuscular mycorrhizal fungi improves establishment of micropropagated Leucaena leucocephala plantlets

Investigations were carried out to achieve cent per cent transplantation success of micropropagated Leucaena leucocephala (a fast growing multipurpose leguminous tree species) plantlets using two vesicular arbuscular mycorrhizal fungi, Glomus fasciculatum and Glomus macrocarpum. Plantlets were obtained by rooting the shoots [obtained through; hypocotyl callus in presence of 10^-5M BAP + 10^-6M NAA; and axillary bud sprouting from cotyledonary and other nodes in presence of 10^-5M BAP, on Gamborg's B5 medium], on half strength B5 medium supplemented with 5×10^-6M IBA. Subsequent to the nodulation of their roots with Rhizobium (strain PRGL 001)in soilrite, these plantlets were tranferred to sterilized garden soil by laying inoculum of either Glomus fasciculatum or Glomus macrocarpum around their roots. Only 20% of the plantlets survived in soils lacking VAM fungus. In contrast, cent per cent of the plantlets of Leucaena leucocephala established very well and showed good growth in VAM inoculated soil. Roots of the later plantlets showed presence of both external and internal hyphae with well formed arbuscules and vesicles confirming the establishment of good mycorrhizal association. These studies convincingly demonstrate that the mycorrhizal association help in successful establishment of tissue culture raised plantlets of Leucaena leucocephala in the field conditions by alleviating the transplantation shock. This revised version was published online in June 2006 with corrections to the Cover Date.     

Molecular identification and phylogeny of arbuscular mycorrhizal fungi

The fossil record and molecular data show that the evolutionary history of arbuscular mycorrhizal fungi (Glomales) goes back at least to the Ordovician (460 million years ago), coinciding with the colonization of the terrestrial environment by the first land plants. At that time, the land flora only consisted of plants on the bryophytic level. Ribosomal DNA sequences indicate that the diversity within the Glomales on the family and genus level is much higher than previously expected from morphology-based taxonomy. Two deeply divergent lineages were found and described in two new genera, Archaeospora and Paraglomus, each in its own family. Based on a fast-growing number of available DNA sequences, several systems for molecular identification of the Glomales within roots have been designed and tested in the past few years. These detection methods have opened up entirely new perspectives for studying the ecology of arbuscular mycorrhiza.     

Phosphate transporters of Medicago truncatula and arbuscular mycorrhizal fungi

Most vascular plants acquire phosphate from their environment either directly, via the roots, or indirectly, via a symbiotic interaction with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the plant roots where the fungi colonize the cortex of the root to obtain carbon from the plant host, while assisting the plant with acquisition of phosphate and other mineral nutrients from the soil solution. As a first step toward understanding the molecular basis of the symbiosis and phosphate utilization, we have cloned and characterized phosphate transporter genes from the AM fungi Glomus versiforme and Glomus intraradices, and from the roots of a host plant, Medicago truncatula. Expression analyses and localization studies indicate that each of these transporters has a role in phosphate uptake from the soil solution.     

Development and amplification of multiple co-dominant genetic markers from single spores of arbuscular mycorrhizal fungi by nested multiplex PCR

Multiple co-dominant genetic markers from single spores of the arbuscular mycorrhizal (AM) fungi Glomus mosseae, Glomus caledonium, and Glomus geosporum were amplified by nested multiplex PCR using a combination of primers for simultaneous amplification of five loci in one PCR. Subsequently, each marker was amplified separately in nested PCR using specific primers. Polymorphic loci within the three putative single copy genes GmFOX2, GmTOR2, and GmGIN1 were characterized by sequencing and single strand conformation polymorphisms (SSCP). Primers specific for the LSU rDNA D2 region were included in the multiplex PCR to ensure correct identification of the Glomus spp. spores. Single AM fungal spores were characterized as multilocus genotypes by combining alleles of each amplified locus. Only one copy of each putative single copy gene could be amplified from each spore, indicating that spores are homokaryotic. All isolates of G. mosseae had unique genotypes. The amplification of multiple co-dominant genetic markers from single spores by the nested multiplex PCR approach provides an important tool for future studies of AM fungi population genetics and evolution.     

丛枝菌根真菌侵染势与接种势之间的关系

丛枝菌根(AM)真菌的侵染势(Colonizationpotential,CP)和接种势(inoculumpotential,IP)是菌根学领域非常重要的两个概念。IP已定义为接种物中有活力的真菌繁殖体及结构的数量(Liu&Luo,1994)。而CP的定量描述和测定方法尚未建立。本文将CP定义为单位数量接种物在侵染初期侵染植物根系的能力,其定量测定公式为:CP=N×L/IP×T,其中N为单位根长侵入点数+根内和根外菌丝数+含有丛枝的细胞数+泡囊数;L为每株寄主植物根系总长度;IP为接种物的接种势单位数;T为接种后的天数。用棉花(Gossypiumhirsutum)、大豆(Glycinemax)、红三叶(Trifoliumpratense)和玉米(Zeamays)和3种AM真菌Gigasporamargarita(Gim),Glomusintraradices(Gi),andGlomusversiforme(Gv)不同剂量(100,300,900,2700and8100接种势单位)的接种物进行试验,以定量测定CP、以及CP和IP之间的关系。结果表明,在相同数量的IP条件下,不同AM真菌具有不同的CP,应用该研究…     

Alleviation of drought stress of marigold (Tagetes erecta) plants by using arbuscular mycorrhizal fungi

The effect of an arbuscular mycorrhizal fungus “AMF” (Glomus constrictum Trappe) on growth, pigments, and phosphorous content of marigold (Tagetes erecta) plant grown under different levels of drought stress was investigated. The applied drought stress levels reduced growth vigor (i.e. plant height, shoot dry weight, flower diameter as well as its fresh and dry weights) of mycorrhizal and non-mycorrhizal plant as compared to control plant (non-drought stressed plant). The presence of mycorrhizal fungus, however, stimulated all growth parameters of the treated plant comparing to non-mycorrhizal treated plant. The photosynthetic pigments (carotene in flowers and chlorophylls a and b in leaves) were also stimulated by the mycorrhizal fungi of well-watered as well as of water-stressed plants. The total pigments of mycorrhizal plants grown under well-watered conditions were higher than those of non-mycorrhizal ones by 60%. In most cases, drought-stressed mycorrhizal plants were significantly better than those of the non-mycorrhizal plants. So, the overall results suggest that mycorrhizal fungal colonization affects host plant positively on growth, pigments, and phosphorous content, flower quality and thereby alleviates the stress imposed by water with holding.     

Genetic variability in a population of arbuscular mycorrhizal fungi causes variation in plant growth

Different species of arbuscular mycorrhizal fungi (AMF) alter plant growth and affect plant coexistence and diversity. Effects of within-AMF species or within-population variation on plant growth have received less attention. High genetic variation exists within AMF populations. However, it is unknown whether genetic variation contributes to differences in plant growth. In our study, a population of AMF was cultivated under identical conditions for several generations prior to the experiments thus avoiding environmental maternal effects. We show that genetically different Glomus intraradices isolates from one AMF population significantly alter plant growth in an axenic system and in greenhouse experiments. Isolates increased or reduced plant growth meaning that plants potentially receive benefits or are subject to costs by forming associations with different individuals in the AMF population. This shows that genetic variability in AMF populations could affect host-plant fitness and should be considered in future research to understand these important soil organisms.